Did my Time Machine backup copy over properly? I have an old WD hard drive that I used for Time Machine for a while, but the drive is showing signs of age (it starts buzzing loudly if left on for a while), so I bought a new (Toshiba) drive to replace it. I had on the WD hard drive a Time Machine backup for someone else's computer also that I didn't really need anymore. The total size between the two backups was about 300 GB (100 for theirs, 200 GB for my computer). Anyway in the morning I dragged the backups folder with both backups from the WD to the Toshiba. Took like 5 hours for it to finish "preparing to copy", and by the time I wanted to go to bed it still had like 70 GB left to copy. I didn't want to leave it running so I hit cancel, figuring that the next day I could delete the other computer's backup and try again. So today I was going to do that, and I saw it actually had already copied my computer's backup to the Toshiba HDD and part of the other computer's Time Machine backup. It seemed like it did the folder for my computer first and then moved on to the backup of the other computer. But when I compared the folder for my computer's backup between the new and old hard drives, the old folder showed about 300 MB larger than the new one. It's pretty close but it makes me wonder.

So I set the new HDD as the Time Machine drive, Time Machine seems to work normally, it even did a backup to the new drive like normal. Does it sound like it copied properly? Can size variations like that between large directories of copied folders happen, maybe there are certain unnecessary things that don't copy, or size is read differently? I would very much like to avoid wiping the backup off the new hard drive and re-copying everything.

Secondarily, if it's possible that some files in the backup didn't transfer, would the fact that it did a new backup onto that drive fill in any gaps or missing files if I needed to, say, restore from a time machine backup?
Time Machine does not make a 100% perfect copy, nor does it necessarily make a BOOTABLE copy should you need to swap it in suddenly one day.  A better way to upgrade to a new SSD (don't spend all that time only to put in another slow spinning drive that only lasts 2-3 years these days!), is to also purchase a usb-to-sata transfer cable for $10-$20, use Carbon Copy Cloner (CCC) on the old mac to make a bootable mirror image, including the hidden recovery partition (and possibly update it!), then swap drives and go about your life.  Optionally wipe the old drive to use for testing or backups if it's still safe to use at all.. likely not if it's several years old.  CCC is a fantastic paid backup app, but I think you can get away with making a full bootable clone without buying it first.  This may have changed.