Well, depending on the drive and if this 40gb of loss was one file and how quickly he shut down after saving the data, it actually is possible. Some drives will use write cache to speed up the perception of the write speeds of a drive where new data is very quickly written to a faster cache, then is transferred a little more slowly to permanent storage. But this write cache isn’t always power loss protected. If you do a normal shut down, the computer waits until any data on the write cache gets transferred to permanent storage before it fully shuts down. If you just nix the power though, that data could be gone, and if it was part of a larger file, it would corrupt that file.
Well, depending on the drive and if this 40gb of loss was one file and how quickly he shut down after saving the data, it actually is possible. Some drives will use write cache to speed up the perception of the write speeds of a drive where new data is very quickly written to a faster cache, then is transferred a little more slowly to permanent storage. But this write cache isn’t always power loss protected. If you do a normal shut down, the computer waits until any data on the write cache gets transferred to permanent storage before it fully shuts down. If you just nix the power though, that data could be gone, and if it was part of a larger file, it would corrupt that file.
Edit: Here’s a source to back up my info. Though, it looks like it may actually be an os feature instead of a drive feature. https://www.iolosystem.com/resources/disk-write-caching.html