External hard drive backup after Big Sur

Hi Community gurus. I've an early 2014 MacBook Air (hanging on for the new Silicon Pro) and have always used a Freecom Mobile Drive (1TB) for regular backups. Just tried to run a backup for the first time since moving to Big Sur and Time Machine is saying "Time Machine couldn't complete the backup to My Passport for Mac" and "The backup disk could not be found. Make sure the backup disk is connected or select a different backup disk." Looking in Finder the disk is there showing as the three partitions - mine plus 2 more (I used it a couple of years back on the kids school laptops before they had their own backups). I'm sure it never used to split it by partition in Finder (but could be wrong). Looking in "my" partition is doesn't show any backups since 2014 - I've only had the disk a couple of years... Any ideas how I can fix this so a) I haven't lost all the recent backups and b) I can still actually back up my current work? Many thanks, D