Guide
How to migrate files to a new drive without losing anything.
Data migration is one of the moments with the highest real risk of file loss — not because the new drive fails, but because of avoidable human errors during the process.
Before you start: the golden rule
Never delete or reformat the original drive until you've verified, file by file or at least folder by folder, that the copy on the new drive is complete and functional. This seems obvious, but it's exactly the step people skip when they're in a hurry — and it's the most common cause of data loss "during a migration," which actually happens afterward, once the original drive has already been wiped prematurely.
Step 1: inventory before moving anything
Before copying a single file, know exactly how much space what you're migrating actually takes up. Check the total size of your main folders and compare it against the new drive's capacity — don't assume "it'll fit," verify it. If you're unsure how much space you'll need going forward, the capacity by device guide helps you size it with margin.
Step 2: copy, don't move, on the first pass
Use "copy" instead of "cut/move" for the initial migration. This keeps the original files intact on the old drive while the copy completes — if something fails halfway through (power outage, system error, drive accidentally disconnected), you won't have lost anything, because the original is still there. Only consider deleting from the old drive after verifying the complete copy.
Step 3: verify before you trust it
Compare the file count and total size between source and destination — most operating systems show this information in a folder's properties. If the numbers don't match exactly, something didn't copy completely, and it's worth investigating before continuing. For especially important files, open them directly from the new drive to confirm they aren't corrupted, not just that they "exist."
Step 4: test with real use before retiring the old drive
Use the new drive as your primary storage for a reasonable period (a few weeks) before considering deleting data on the old one. This gives you margin to discover files you might have missed, compatibility issues, or any surprise that isn't obvious in an initial quick check.
A common mistake: trusting copy speed as a sign of success
A copy "finishing" without visible error messages doesn't guarantee every file transferred correctly — some permission errors or in-use files can be skipped silently without a clear warning depending on the program you use. You can estimate how long your migration should reasonably take based on size and connection speed with the file transfer calculator — if the process finishes much faster than expected, that's a sign something may have been skipped.
Verifiable sources
The recommendations in this guide are consistent with official data migration processes documented by Microsoft and Apple for their respective operating systems, which also insist on verifying the copy before deleting the source.