nostr:npub1wstchnnatqhe4ua983uhsn7k0mlvxlrdl8nfjledgr6ez7hcppdspcez75 Since I would be mounting and unmounting I'm pretty sure I could just pick the mount point. The directory names would always be consistent.
nostr:npub1klw906w2f0lwfp7fle9al80fe77n7nmgnlyfh6s4he8gvn09t9dql0jmy2 is it always the same drive indicator? i.e. /dev/sdb or whatever? If so, you could use an if statement to see if that directory exists.
If the drive letters change, then you could use fstab to always mount it in the same directory. Then use an if statement to check to see if that directory has contents.
Discussion
nostr:npub1klw906w2f0lwfp7fle9al80fe77n7nmgnlyfh6s4he8gvn09t9dql0jmy2 Then an if statement is probably the way to go. Just wrap the entire rsync command in the if statement and send out an error if the directory doesn't exist.