time machine can not recognize new disk nor find old disk (says waiting to complete)

jdpdx46 wrote:

Bought a new macbook pro. Used Time Machine b/u to restore data. Mostly fine.

Attempted to use time machine to b/u again. Took a long time. Next morning said didn't finish. Can't use disk again. Says didn't finish. No other slot available. Removed oldest slot. Still no slot available. Trying to use new disk. Now what?

You make no mention of the old Mac or what macOS was running or how the external drive was formatted...

A new Mac , a new file system, I would start over with that external drive if you want to use it for TM going forward—

Erase and reformat a storage device in Disk Utility on Mac

https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/22.0/mac/13.0

The time when you move to a new Mac is THE most diabolical SPACE issue in usage of Time machine. It is the source of the recommendation that you have 2.5 times the backup space as you have files to back up.

What needs to happen after you change Macs is that you need space for a Full Backup of everything on the new Mac. At exactly the same time you need All files form the old Mac. so one times the size of all files, plus one times the size of all the old files PLUS older parts of older backups, estimated at another half as much.

if enough space is not available, then Time machine automatically tries to make more space available. The problem is, it can't just willy-nilly delete old files, because they are linked to newer backups (so that you don't store two copies). So it has to Consolidate and Thin oldest backups, and eventually they can be removed automatically. This takes a very long time.

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The approach that does not leave you "working without a net" is attaching an ADDITIONAL backup drive, and letting Time machine start a new, stand-alone backup set on the new drive. Each drive will contain a stand alone backup SET starting with when it was created, thinned down as space gets tight. Every-other backup goes to every-other drive.