One of my backup drives failed and I need to upgrade from Catalina to Big Sur

I’ve never had an issue with it, (As far as the formatting), so are you saying it’s already formatted as HFS+?

That should be just fine, although if you already have data on this drive, and plan on using it for a new Time Machine drive, I’d backup the data on this drive, reformat again to use with Time Machine, as it’s not recommended to use an external drive for both Time Machine and other storage. Although creating a new partition on the drive with the same formatting should be fine.

Here, if you haven’t seen these article yet, check them out, very informative:

Types of disks you can use with Time Machine on Mac:

https://support.apple.com/guide/mac-help/types-of-disks-you-can-use-with-time-machine-mh15139/mac

Time Machine troubleshooting on Mac:

https://support.apple.com/guide/mac-help/time-machine-troubleshooting-mh15653/11.0/mac/11.0

Back up your Mac with Time Machine:

https://support.apple.com/en-us/HT201250

Hope one of those shed’s some light on it!

Tips How to Upgrade to Big Sur 

There are several points to consider before attempting the upgrade to Big Sur. The most important is Due Diligence on the part of the User and is a Prerequisite for a smooth upgrade.  

The below points should all be consider Before attempting the upgrade.

Point # 1 - Are there pre-existing Unresolved Software Issues. They should be resolved prior to upgrading - they may carryover to Big Sur making the system unstable / unusable.

Point # 2 - Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.

Point # 3 - What are the System Requirements ie: Empty Space Requirements specific to the Big Sur.

Point # 4 - Does the intended computer qualify for the upgrade.

Point # 5 - Are the installed Applications / Extensions compatible with Big Sur. Check with the Developer for compatible Apps / extension or do they offer updates. Same goes for Third Party Drivers for Printer, Scanners etc.

Point # 6 - If upgrading from Mojave and earlier versions of macOS - Remember that Big Sur does not support anything 32 bit. These include Applications, Extensions and Drivers for any devices. There is no workaround.

Point # 7 - Do I have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Point # 8  - To upgrade to Big Sur will require a quick, stable and reliable Internet Connection - preferred method is via Ethernet Connection. If not available and using Wifi - move computer as close to the Access Point / Router and disconnect all other devices connected to Wifi. Want the maximum bandwidth for the computer being upgraded to Big Sur.

Hey there!

Good question!

If your iMac can support Big Sur, then it shouldn’t matter which format your new external drive is.

I’ve learned from my own experiences (And others who post in here regurlarly), that APFS should be preserved for SSD’s and HFS+ (Journaled) for HDD’s.

However I’ve also backed up to SSD’s formatted as HFS+, and have had no issues.

My experience is Mac OS Extended journaled (HFS+) is always a safe bet regardless, however, I imagine as the Mac formatting evolves APFS may eventually be the better bet, although as of now, on typical external hard drives, HFS+ still works without a hitch.

Big Sur can use either format. If you start with a clean drive with Big Sur TM will auto format to APFS not matter SSD or mechanical. If you start with a used drive that has data on it formatted HFS it will keep it. Not to worry.