Migration Assistant incorrectly flags "not enough free space"

It may in fact be a bug in Migration Assistant, but if it is, there is nothing anyone can do about that right now, other than reporting it to Apple as I'll suggest later.

My old device had 247GB of data stored on its hard drive, ...

Disregard that figure because it is inaccurate at best. Different macOS versions have used different methods of calculating and / or reporting it. The only detail that matters is its storage capacity. The replacement Mac must have at least as much. Does it?

If the migration fails contact Apple and have them get to the bottom of it. Initial setup assistance is a complimentary service included with purchasing new Macs from Apple. You paid for it already, so why not take advantage of it.

Apple would have sent an email to the address you used to purchase that Mac, but if you did not receive it, just use the Support link on these pages to get started.

If you actually had 247 GB of data on your old Mac, a new Mac with only 500 GB of storage space is not nearly big enough.

You have failed to allocate anything to the minimum 10 percent or more of required FREE space required to operate without crashing.

The largest item that needs that space is Virtual Memory backing store. If you ever use more than your actual Real RAM, the Mac needs to use disk space for virtual memory. You could easily need an additional 64 GB or Virtual RAM storage in an instant.

Ignoring that need makes your new Mac operate very slowly before it crashes, occasionally in a very spectacular way.

No, it won't work.

Replacing a Mac with one that has half the original Mac's storage capacity is not a good idea. Neither is purchasing a used Mac.

If "Backmarket" has a return policy similar to Apple's that's what I recommend.

hydrolaze wrote:

I made what seemed like a logical decision to trust the Mac storage stats

Appearances can be deceiving.

I bought it secondhand from Backmarket

Maybe you should have led with that.

I didn't respond to your message at first. It didn't make much sense. 247 GB of data should fit on 500 GB with no problem. But you did mention "storage analysis tool in settings", so who knows. Storage can be horribly complicated for many different reasons.

However, you've since added an entirely new level of complexity to the question. A significant number of problems reported in these forums start with a similar statement. Never, ever buy a used Mac - not under any circumstances. You will regret it.

If you want to save some money, buy a certified refurbished device from Apple. But if you aren't buying a refurbished device from Apple, then you're buying a used device. Maybe it's functional. Maybe it's as described. Maybe it already has 250 GB of data hidden away somewhere on it.

John Galt wrote:

If "Backmarket" has a return policy similar to Apple's that's what I recommend.

Excerpted from https://help.backmarket.com/en-us/articles/844924-what-if-i-change-my-mind-about-the-item-within-the-first-30-days

"If you change your mind about your item, you have a period of 30 days (starting from the date you received the item) to start a return by contacting Back Market Customer Care. In this case, the seller must refund you for the returned item."

That's even better than Apple's return policy. Be sure to follow their return instructions to the letter, lest they deny your request:

https://help.backmarket.com/en-us/articles/844921-how-do-i-return-an-item-i-bought-on-back-market

etresoft wrote: Never, ever buy a used Mac - not under any circumstances. You will regret it.

I have purchased used Macs, and I have sold them, and everyone has always been happy. For that reason I always stopped short of taking such a hard line, stating "purchasing a used Mac is best left for very experienced Mac users" instead. After hearing too many of these sad tales I'm not doing that any more. Never buy a used Mac.