A lot of people have this problem, so did I.
When copying large files the PlayonHD samba server hangs.
Tried everything and then figured it out.

It is related to SMB 2.0 and security settings which are incompatible with SAMBA.
Not just the playon but more NAS devices have these similar problems.
The problem is client related. XP is fine but Vista and Windows 7 have these problems because of higher security settings.
This is fairly to change by lowering these settings in the local policies.

The workaround per client:
Start local policy editor: Start, Run, GPEDIT.MSC
Local Computer Policy, Computer Configuration, Windows Settings, Security Settings, Local Policies, Security options:
- Network security: LAN Manager authentication level = Send LM & NTLM responses
- Network security: Minimum session security for NTLM => Disable: Require 128-bit encryption

After that: NO SAMBA ISSUES.
Samba just cannot handle the new authentication and encryption protocols.
My readyNAS works smoother now too.

Thank you. I moved your post to the How To section.

I have implemented these changes in Windows 7 Home Premium by editing the registry as shown below. However, I have not noticed any change in the Samba stability. In other words, the AC Ryan NAS drive still drops off my network after some time. I have only implemented the change for the past day and I haven't done extensive testing, but I have seen the same problem as before so far.

I'm not entirely sure this How To is meant to fix the Samba stability problem. I think if if truly did fix the problem, then all the dissatisfied PlayonHD owners would be happy now.

Here is how I edited the registry, which is information I found elsewhere on the web related to making these changes to improve Samba performance in Win7.

LM Authentication Level registry edit procedure and codes shown http://www.sevenforums.com/network-sharing/202099-secpol-msc-network-security-workaround.html and http://www.tannerwilliamson.com/2009/09/14/windows-7-seven-network-file-sharing-fix-samba-smb/(same fix, different blog)
Go into regedit and Navigate to:
HKLM|System|CurrentControlSet|Control|LSA
If the key LMCompatabilityLevel is present, set it's value to 1.
If not present, add a new DWORD (32 bit value), and set its value to 0x1 (Hexadecimal 1).
Reboot. And see how you go. Test your system to see if changing this setting impacted other clients or services.
If this does not work, I'd return the registry to the way it was before.
When LM Authentication level is not defined the keyword is generally not present in the registry.
The values are;
0 = Send LM & NTLM responses
1 = Use NTLMv2 responses if negotiated
2 = Send NTLM response only
3 = Send NTLMv2 response only
4 = Send NTLMv2 response only. Refuse LM
5 = Send NTLMv2 response only. Refuse LM & NTLM

Other Samba registry changes described http://www.sevenforums.com/network-sharing/3737-samba-network-share-issue-windows-7-a-3.html
Used the LM+NTLM security setting (from above), and also the settings in the registry (Windows7 - SambaWiki)
HKLM\System\CCS\Services\LanmanWorkstation\Parameters
DWORD DomainCompatibilityMode = 1
DWORD DNSNameResolutionRequired = 0