Mac mini M6 running macOS 27 not recognizing external drives for Time Machine

Open Applications > Utilities > Disk Utility, select View menu > Show all devices. Does it show up there, and if so, can you mount it (button in the top toolbar)?

More info: Disk Utility User Guide for Mac - Apple Support

If you have 3rd party ‘protection’ software installed, that may be the problem. For example, ESET has been reported to prevent external drives and memory cards from mounting. Such 3rd party system ‘helpers’, ‘cleaners’ and ‘protectors’ do more harm than good. In fact, such products are completely unnecessary for macOS and the only ‘good’ (if you want to call it that) they do is to put money in the developers’ pockets. For the user, they offer only harm…and if that is limited to harm to your bank account, then count yourself lucky.

There were a few prior posts on this forum from users who couldn’t access their external drive due to the drive manufacturer’s software that they used to encrypt the drive. Lacie/Seagate, Samsung, SanDisk, etc., all provide 'drive management' software on their drives and using such 3rd party software can cause issues when macOS is updated and the 3rd party software becomes incompatible. Not sure if that's the issue here, but FYI the best practice with a new external drive (SSD/HDD, not applicable to NAS drives) is to use Disk Utility to complete erase it, format it as GUID/APFS and apply encryption if desired.

If your drive is using a FireWire connection, it will not work anymore because starting with macOS 26 Tahoe, FireWire has been fully deprecated by Apple and is no longer supported.

FWIW, I have a bunch of Samsung T7 and T9 SSDs ranging from 1 TB to 4 TB and a pair of Crucial 8 TB SSDs, and they all work fine with my M4 Pro MacBook Pro running the current version of macOS (27.2).

MacGolferMx wrote:

The one disk physically connected to the map is a FireWire device,

My second disk is an old Apple's Time Capsule (which is working perfectly fine), so the way I connect to it is via an Ethernet cable. As far as I know, I cannot use Disk Utility to "format" it to ensure that it is APFS. Or Can I?

If you know a way to do this I'll be more than grateful to know.

Knowing the disk types initially would have helped.

Time Capsule will no longer work starting with macOS 27 Golden Gate. It uses a deprecated format for networking (Apple Filing Protocol, AFP), and while that format works with macOS 26 Tahoe, the format will no longer be supported by Apple starting with macOS 27. Apple has exited the networking hardware market. It has been over 7 years since the Time Capsule was last sold by Apple, and that means the product is now considered 'obsolete'. To put it bluntly, your Time Capsule is now essentially a paperweight. In theory, you can physically disassemble the Time Capsule, extract the hard drive and use it in an external enclosure. IMO, not worth the trouble.

The simplest method to continue using Apple’s Time Machine to back up your Mac(s) is to buy a modern external drive and connect it to the Mac(s) regularly to back them up (or leave the drive connected to a desktop Mac). External storage is relatively inexpensive, a 1 TB SSD or a 2-4 TB HDD can be purchased for <$200 (prices have gone up significantly in recent months, unfortunately). 

To back up your Macs over WiFi, the most straightforward way is to use a network attached storage (NAS) device – essentially a box for hard disks that connects to your network. Ideally, choose one that offers turnkey support for Time Machine out of the box, both Synology (setup instructions) and QNAP (setup instructions) do so. Technically, you can stand up a Time Machine server in other ways (including using a Mac as a TM server), but an NAS is arguably the most straightforward way to set up network Time Machine backups. However, since you have that old MBP running Sonoma, standing that up as a TM Server might be an option.

Personally, I use a Synology DS223j with a pair of 10 TB HDDs installed to back up 5 Macs in the house with Time Machine (separate shared folder for each Mac sized at ~2.5x the internal storage), with the remaining space used as general file storage for everyone in the house. 

It still bothers me that just by upgrading to Golden Gate I have to go through all this hassle and even consider spending a few extra hundred dollars to operate as I was before.

That's technology. I still have one of these somewhere in my attic.

I mean, one of those 100 MB removable Zip disks would only hold about three RAW images from my current camera, but still... 😉

MacOS Golden Gate does not support Time Capsule, or the HFS+ drives hanging off an AirPort Extreme Base Station for backups. Neither of those Apple devices understand the APFS drive formatting required by the current Time Machine application, and if those drives are that old, seriously consider replacement. This is getting ahead of drive failure.

My Time Machine configuration uses a USB-C, direct-connected APFS formatted drive to my M4 Mac Mini Pro. That configuration will allow Time Machine to dutifully add the drive, and begin a backup.

I happen to use a 2 TB Crucial SSD that was already in my possession before the astronomical SSD price increases over the last year. You can get by with a appropriately sized 7200 RPM rotational drive with 256MB cache in an external USB-C 3.2 drive enclosure.

Time Machine drives need to have the entire volume formatted as APFS and a GUID partition scheme. I don't do encrytion as the backup is slow enough without it.

Visit System Settings > Time Machine > and click the + symbol to add your mounted Time Machine drive. Under Options, set Backup Frequency to Automatically every hour. That is the frequency I have been using for years.

In System Settings > Menu Bar, select Time Machine to put it in your menu extras area so that when you click on it, you can initially start a full backup, and see when the last backup has completed.

With rotational disks, the first backup will take awhile.

Thanks for your reply.

No 3rd party encryption software in my case.

The one disk physically connected to the map is a FireWire device, so at least I now know why I cannot connect to it.

My second disk is an old Apple's Time Capsule (which is working perfectly fine), so the way I connect to it is via an Ethernet cable. As far as I know, I cannot use Disk Utility to "format" it to ensure that it is APFS. Or Can I?

If you know a way to do this I'll be more than grateful to know.

It still bothers me that just by upgrading to Golden Gate I have to go through all this hassle and even consider spending a few extra hundred dollars to operate as I was before.

Thanks again for your help.

Thanks for your suggestions.

The fact is that Time Machine does not allow me to set up any of my disks as backup disks, therefore cannot set up backup frequency or any other parameter for that matter.

Thanks anyway

Thanks again for your prompt reply.

Hard to hear, but at least I will stop trying.

I will start looking for new external ssd options.

Thanks again.

I am already looking for options.