How do I force Time Machine to back up to a USB HDD, instead of a .sparsebundle on that USB HDD?

I use an iMac as an external monitor for my MacBook pro (OS X 11.6), over a thunderbolt cable. I usually have a USB 3 hard drive attached to my MacBook pro as a Time Machine backup. Last week I thought I would try attaching my USB drive to my iMac, and mounting it as a shared network drive over the thunderbolt network bridge. Of course, Time Machine then created a .sparsebundle disk image to perform the backup over the network. I decided I didn't want to start a full backup from scratch, and so moved the USB drive back to a direct connection.

However, now Time Machine only wants to back up to the .sparsebundle image, even thought the drive is connected locally and has a full set of backups at the root of the drive (in Backups.backupd). I tried using tmutil setdestination and tmutil associateddisk -a to force the association, with no success. I even renamed the .sparsebundle, and every time I start a backup the .sparsebundle is renamed back to the old name.

How can I stop Time Machine from treating the USB-attached hard drive as a network share for backups?

In order to prevent Time Machine from continuing to use the created .sparsebundle for its backups, you need to delete it.

Time Machine tries to backup to .sparsebundle files first. If it can't find any, it will create regular local backup.