G Slim drive showing in Finder and Disk Utility - but cannot back up my files from Macbook Pro

That drive now appears to be formatted correctly for this use.

now connect it as a Time Machine backup destination.

Connect a new backup disk to your Mac - Apple Support

Then try to back up, and report the Exact error message that appears.

YOu are reformatting the parent drive (top most) From DiskUtility>View>Show all Devices

I would erase/reformat/initialize again as new, and try again.

GUID / APFS encrypted

Back up your Mac Back up your Mac with Time Machine - Apple Support

Here is an Apple article with instructions for erasing the whole physical drive:

https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac

Unfortunately Disk Utility hides the physical drive from view by default and it is critical to erase the whole physical drive to ensure an external drive is properly partitioned & formatted.

<< Did you look at the screen grab that I included ? >>

Yes!

That shows only the Volume that a person named as G-Drive Slim and

clearly identifies that it is very an APFS Volume, NOT a Disk.

the tiny View menu MUST be set to "show all devices" for success.

this is what I used to get from "Show only Volumes"

Seeing only well-behaved names that humans created is a very strong clue you are not in the right place yet.

Seeing VOLUME as the description of exactly your item will not bring you the success you seek.

as a holdover from previous versions of MacOS, Disk Utility pretends there is no complexity, and shows a sanitized view by default.

You need to use the tiny "View" menu set to 'Show all devices'. Then ERASE the DEVICE (not just the Volume) by its immutable manufacturer-given device-name:

See that awkward, wonky device-name, "WDC WD740ADFD-00NLR4.. ??

Says '(Internal) Physical Disk' so right under the name.

To fix the "Backup disk not available" error with Time Machine or file backups on macOS Sequoia, your G Slim drive needs to be reformatted using the correct APFS file system (or Mac OS Extended with a proper partition map, though APFS is required for modern Time Machine backups) and selected properly.

How large is your internal disk which you are trying to backup? That 500GB disk is too small for any backup.

Use a storage device with at least twice the storage capacity of your Mac. For example, if your Mac has 1TB of storage, your backup disk should ideally have at least 2TB of storage. If Time Machine needs more storage space, your Mac will let you know.

Hi,

Since your G Slim drive appears in both Finder and Disk Utility, please check the following:

Open Finder, select the external drive, and choose Get Info.

Check the drive format and available storage space.

If the drive uses NTFS, macOS may be

able to read it but not write files to it.

Check whether the drive is reserved for Time Machine backups.

If necessary, use Disk Utility > First Aid to check for file system errors.

Important: Do not erase or reformat the drive before safely backing up any existing data.

Could you confirm the drive’s format and the exact error message you receive when trying to copy files?

1. Hello, thank you for trying to help me. I have tried several times to do this but without success. I really need step by step instructions. Even Apple's own instructions on how to do this are not helpful. For example Apple suggests : Click the Scheme pop-up menu, then choose GUID Partition Map. - But there is no Scheme pop-up menu. It then states : If the Scheme pop-up menu isn’t available, make sure you selected the storage device you want to erase — it’s at the top of the hierarchy. But that is incorrect. The storage device at the top of the hierarchy is my Mac HD and I do NOT want to mess with that. I want to be able to back up to my external HD called G Slim. It then states : Click the Format pop-up menu, then choose a file system format. But there is NO format pop-up menu. Aaagh ...
2. I have included a screen grap of my disk utility. G drive slim appears in the side bar but I cannot back up to it. Please help - with clear instructions even a 5 year old child can follow. Thanks in advance
3. 

Hey Grant Bennet-Aider,

Yes, I do see that "awkward, wonky device name ...." Did you look at the screen grab that I included ? At the top of the Disk Utility side bar it lists my Internal Mac Hard Drive. I do NOT want to mess with that. Below is the external hard drive. I did already erase it as per Apples instructions - https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac. The problem is that now it has been erased I cannot back up my Mac to the External HD G Slim. Disk Utility can see the External G Drive slim, it lists it in the side-bar menu but when I select Back up it states that the G Slim drive is not available. Notice that on your screengrab - you have little down arrows next to the wonky device name. On my screengrab and disk utility there aren't any down arrows. So I cannot access the G drive Slim. So any other suggestions, please ?

No I am not reformatting my Internal Mac hard drive, whichi is listed at the top of the hierarchy on the Disk Utility. I did erase the External Hard drive called G Slim, according to Apples instruction https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac

Now the External hard drive G slim is shown in Disk Utility - therefore Disk Utility can see it. But when I try to back up my Mac to the external drive G slim my Mac states external hard drive G slim is not available. I've included screenshots of my Disk Utility. Any suggestions please ?

Womac23 wrote:

No I am not reformatting my Internal Mac hard drive, whichi is listed at the top of the hierarchy on the Disk Utility. I did erase the External Hard drive called G Slim, according to Apples instruction https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac

Now the External hard drive G slim is shown in Disk Utility - therefore Disk Utility can see it. But when I try to back up my Mac to the external drive G slim my Mac states external hard drive G slim is not available. I've included screenshots of my Disk Utility. Any suggestions please ?

You need to make sure to perform Step#2 in the linked Apple article in order to have Disk Utility show you the physical drives on the left pane of Disk Utility. That is why I provided that link.

Here is an older version archived on the Internet Archive which also includes some graphics which may help here:

https://web.archive.org/web/20250909095655/https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac

Hello Grant Bennet-Aider,

Thank you for your patience and continuing support. I clicked on the tiny 'View' menu and selected 'Show all devices'. I could then see what you were referring to. I selected the USB External Physical Disk with the very long name HGST HTS ......... Media. Next I selected the Erase icon. It asked me to input a name so I added CH. It then created another 'device - not sure if that is the correct terminology' with the name CH and APFS Volume and showed 499,9 GB capacity. I then tried to perform a back up from my Mac to the USB external hard drive - again without success: 'Back up disk not available' . I checked the instructions, again, on the Apple erase and reformat guidelines. And I repeated the exercise this time using name CH APFS volume. Again without success. I include two screengrabs here. What am I not seeing or doing please ?