If you plan on reselling a PC hard drive, make sure you thoroughly delete everything inside. A new study from data removal company Blannco reveals that many secondhand storage drives sold on eBay still contain data from the previous owner, including personally identifiable information (PII).

Blannco purchased 159 SSD and HDD storage drives on eBay, and had them analyzed to see if any data could be recovered. The used drives were purchased in the US, UK, Germany, and Finland and covered a range of brands such as Samsung, Dell, and Seagate.

Unfortunately, not all the drives were thoroughly wiped. Sixty-six of them, or 42 percent, still retained some of the data onboard. Of those, 25 drives still held PII, such as photos, birth certificates, names, and email addresses.

"This means more than 15 percent of the drives tested contained sensitive information that could be dangerous in the hands of identity thieves or hackers," Blancco said in the report. "In other words, for every 20 drives, at least three had PII."

One drive, in particular, came from a software developer who had been granted government security clearance. The drive contained his family's birth certificates, scanned copies of their passports, and financial records. Another drive contained thousands of photos from a woman in Denmark, along with her name and friends' names.

"Every seller we purchased drives from insisted that proper data sanitization methods had been performed so that no data was left behind," Blancco said. "This demonstrates that sellers are attempting to permanently wipe data. However, many are failing to use a fully effective solution."

According to Blancco, resellers will attempt to wipe storage drives simply by "formatting" them, believing the process will erase all the data inside. But often, this process only clears the index to reach the files on board the storage drive, and does not erase the data itself. Another problem is that the formatting process doesn't confirm to the owner whether the data is truly gone.

This study echoes what Blannco found in 2016 when it conducted a similar analysis of 200 secondhand hard drives bought on eBay and Craigslist. In that study, 67 percent of the hard drives contained personally identifiable information.

To properly clear a hard drive, Blancco recommends you use its own data erasure products. PCMag also has a guide on other hard drive clearing tools you can use.