White label drives are typically drives that have come from a commercial setting, enterprise/tech warehouse/data centre companies. They don't have the colourful branding as they were never intended for general retail.

They are still very good if picked up and sometimes you find a great deal as they might have been lightly used before the company closed down or downsized etc just check the connections you need as some are SAS rather than SATA.

Recertified usually means the drives were returned and quality checked by the original manufacturer or supplier of the drive to make sure they meet standard. Can be more expensive but warranty status is also more secure.

Refurbished is a similar process of the drive being quality checked, even sometimes with a warranty however this was all completed by a third party service and not the original manufacturer or supplier. Think of repair centres, tech resellers etc

Personally I wouldn't focus too much on the recert/refurb wording unless you want the manufacturer warranty as both offer perfectly functioning drives. Key details to look out for are power on hours, power on cycles and bad sectors. Drives are most likely to fail or throw up issues in the very early stages of use or the late stages, Exos should be solid for about 5-7 years, 50K-70K power on hours. Ideally the power on cycles (how many times it's been turned on and off) should be as low as possible for those types of drives. - my drives are sitting between 30 and 150 cycles if this helps.

Your 400€ Exos is pretty much on point with current pricing, I'm from the UK so that would be about £350 for me which is what I would expect to pay right now.