What technical differences make some flash storage able to retain data better than other?
Here are two flash storage devices in comparison:
Few years ago, a 16 GB USB stick with both USB-A and USB-B-Micro OTG endings by the vendor Hama lost data within months, despite regular usage, which suggests that the firmware does not refresh data to counteract bit fading, flash storage's main weakness. But even then, data should not be corrupted after mere months.
The stick never write-protected itself, and new data could still be written to it while it worked. The new data lasted half a year until the first corruption.
It once stopped working entirely (detected by computer as storage device with no storage inserted) after plugging it into the USB port of a decade-old Grundig boombox CD player, which might have been caused by a voltage spike from poor circuitry, but I am not sure. I will have to test it at some point, but that is the only plausible explanation.
In comparison, MicroSD card from SanDisk, 64 GB, was entirely unused since early 2017 until mid-2020, never powered on, yet still had 100% data integrity, despite having an obviously much higher storage density, meaning smaller transistors. There must have been other physical differences.
I ran a scan using HDDscan, and there were no downspikes in speed. For Linux, there is Gnome Disks, but it has no full sequential scanning mode like HDDScan, only quick scanning by number of samples and size per each sample.
I have had overwhelmingly positive experience with SanDisk's and Transcend's data retention, which suggests me that they refresh its data while idle, to counteract bitfading. Lower-tier brands such as Hama and Integral do not appear to do this. But it can not be the only thing that makes SanDisk and Transcend retain data so well.
Another sign of good quality flash storage may be transfer rate. The Hama stick had 22 MB/s at its peak, even though USB 2.0 allows up to around 33 MB/s, whereas the SanDisk MicroSD reached 84 MB/s. In order to reach such high transfer rate, the memory card must be of good quality.
It could also be that SanDisk's and Transcend's memory cards have more redundant storage used for storing error correction code, or maybe storing all data twice internally.
But what exactly are the differences that make some flash storage retain data much better than other? Anything I didn't mention?