By the way, I hear you can't really "wipe" data on SSD drives. I don't remember why exactly though. Probably some fault-tolerant mechanism. Or maybe it's what NSA has been asking HDD OEMs to make from the beginning.
This means your attempt to write the same address 7 times actually goes to 7 different storage locations and the original data might still be stored wherever it first landed.
(Within an organization at the same security level, deletion and then wipe is enough to re-issue a laptop, but not going from top researcher to intern, and certainly not from inside to outside.)
There are tools called "Secure Erase" that use special functions on the disk to wipe it without wearing it out.
Just write data up to its size. Multiple times, just to be sure
Read it from /dev/urandom
It's certainly better than nothing, but not sufficient practice in a business or high-security-professional environment.
An attacker is willing to load custom firmware onto the drive, or to move the chips into a new device, or otherwise read it out raw, and will have access to more material than your dd can write.
The standard should be "can I prove this will work reliably, given all the layers beneath me", and for that, the only adequate answer is physical destruction. You could possibly design a drive where you're guaranteed to know if everything is fully deleted, and as long as you trusted the design/implementation/current-status, you could rely on it, but then you'd have a $100k 100GB SSD. So much easier just to replace old drives, or to guarantee that nothing interesting every touched the disk unencrypted, or ideally both.
Yes, that's why you fill it multiple times. But yes, you can have information leaking in a sector that went bad and was remapped
But the good thing about SSDs as well is that they're much easier to destroy, just microwave it, or provide an excess voltage to it (may need more work than simply connect the power input to wires coming from the wall)
A lot of SIGINT/COMINT gear designed for field use had magic destructo-capability designed in, too. I think after the USS Pueblo, especially.